The instant invention relates to chemical vapor deposition (CVD) apparatus and methods, and more particularly to a parylene deposition apparatus including an atmospheric shroud enveloping the apparatus, and an inert gas source for providing an inert gaseous atmosphere within the shroud during operation of the apparatus.
Parylene is a general term used to describe a class of poly-p-xylylenes which are derived from a dimer having the structure: ##STR1## wherein X is typically a hydrogen, or a halogen. The most commonly used forms of parylene dimers include the following: ##STR2##
Parylene coatings are obtained from their related parylene dimers by means of a well-known vapor deposition process in which the dimer is vaporized, pyrolized, i.e. cleaved into a monomer vapor form, and fed to a deposition chamber wherein the monomer molecules deposit and polymerize onto a substrate disposed within the deposition chamber. The process occurs according to the following reaction: ##STR3##
Due to their ability to provide thin films and conform to substrates of varied geometric shapes, parylene polymers are ideally suited for use as a conformal external coating in a wide variety of fields, such as for example, in the electronics, automotive, and medical industries.
Octafluoro-[2,2]paracyclophane (Parylene AF4 dimer) is a fluorine substituted version of the above-noted parylene dimers and has the structure: ##STR4## It is known that parylene coatings which are derived from the AF4 dimer by the vapor deposition process have a very high melting temperature (about 500.degree. C.) and a very low dielectric constant (about 2.3). These characteristics make Parylene AF4 ideally suited for many high temperature applications, including electronic applications, and potentially as an inter-layer dielectric material in the production of semiconductor chips. The existing parylene coating systems as used with Parylene C, D, and N, typically include a chamber system comprising a vaporization chamber, a pyrolysis chamber coupled to the vaporization chamber, and a deposition chamber coupled to the pyrolysis chamber in which the monomer vapor deposits onto a substrate and polymerizes. The coating systems further include a vacuum system coupled to the chambers for creating sub-atmospheric pressure conditions throughout the chamber system. While the existing parylene deposition systems are highly effective in depositing parylene C, D, and N, there are unique process characteristics and physical requirements of semiconductor chip manufacturing processes which prevent the existing parylene coating systems from providing sufficient deposition control, uniformity of layer thickness, material efficiency, and speed of coating.
One particular aspect of semiconductor chip manufacturing is that the semiconductor wafers are subjected to high temperature processing operations in the range of 400.degree. C. to 500.degree. C. Accordingly, any dielectric coating material deposited onto the surface of a wafer must be able to withstand these processing temperatures without significant degradation. The presently described Parylene AF4 has been shown to be compatible with the process operations up to the 500.degree. C. levels. However, there is nevertheless a desire to improve the thermal stability of the AF4 polymer to provide a thermal safety buffer and better margin of error.
Accordingly, there is currently presented a need for a parylene deposition system particularly suited to depositing parylene polymers, particularly parylene AF4 polymers, onto semiconductor wafers for use in the production of semiconductor chips. In this regard, the instant invention provides a parylene deposition system comprising a vaporization chamber including a heated and cooled dimer crucible for the vaporization of parylene AF4 dimer, a pyrolysis chamber for pyrolyzing the dimer into its monomer form, a post-pyrolysis chamber for capturing unpyrolyzed dimer prior to entry into the deposition chamber, a deposition bell having a frusto-conical shape to minimize chamber volume and maximize vapor flow, a filter structure positioned at the deposition chamber inlet to filter out microscopic particles and impurities prior to deposition onto the wafer surface, a heated and cooled platen assembly for supporting a semiconductor wafer, an electrostatic clamping device for clamping the wafer in intimate thermal contact with the platen, a quartz crystal deposition rate controller, a vacuum system, and a vacuum by-pass assembly wherein a high-conductance vacuum outlet is utilized to quickly reduce pressure in the chamber system, and a low-conductance vacuum outlet is utilized to maintain vacuum flow through the deposition chamber during the deposition procedure.
The apparatus still further includes an atmospheric shroud which envelopes the entire apparatus, and an inert gas source for providing an inert atmosphere within the shroud. The purpose of the shroud and inert atmosphere is to exclude oxygen from the deposition chamber during evacuation and subsequent coating cycle, thus allowing the coating process to be carried out in a substantially oxygen free environment. The prior art parylene coating devices operate in normal atmospheric conditions. This allows for oxygen to be present as a constituent background gas, and as a constituent of any atmospheric leakage into the chamber during the coating cycle. It has been found that oxygen can chemically combine with the Parylene reactive monomer in the pyrolysis and post pyrolysis zones respectively, and thereby cause weak polymer bonds. Accordingly, the shroud is effective to isolate the apparatus in an inert atmosphere before pump down, and during the coating cycle. Preliminary test results have shown that the inert atmosphere provides a purer, denser and more stable coating.
Accordingly, among the objects of the instant invention are: the provision of a parylene deposition apparatus effective for quick and efficient deposition of Parylene AF4 polymers onto silicon wafers in the production of semiconductor chips; the provision of a parylene deposition apparatus including an atmospheric shroud enveloping the apparatus, and an inert gas source for providing an inert gaseous atmosphere within the shroud; and the further provision of apparatus and methods for the fast, efficient, and cost effective deposition of parylene polymers onto the surface of a silicon wafer.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.